Stop duplicating TypeScript class helpers in every CJS file output

[benlesh]

Currently, we're outputting the class helpers in ever CJS file we output, that means that anyone bundling them will end up having N copies of __extend methods in their bundle, where N in the number of our modules they import that use classes.

This is a particular problem for the Angular team.

cc/ @IgorMinar

[kwonoj]

Check issue #2436, I guess this is dupe.

[benlesh]

this is done